In recent years, an RFID (radio frequency identification) system using a radio signal of a UHF band (for example, 860 MHz-960 MHz) attracted attention. In the RFID system, for example, a radio signal of about 1 W is transmitted from a reader/writer, and a tag receives the radio signal and transmits a response signal to the reader/writer, so that the reader/writer reads information stored in the tag. By the storage of information (ID etc.) into the tag affixed on an article such as a corrugated cardboard and a book, such an RFID system is used as a product inventory management system or a book management system in a library.
A tag for use for the RFID system includes a dipole antenna with a length of 100 mm and a width of 15 mm, for example, and a chip (LSI (large scale integration), for example) with a length and a width of 1 mm or less, respectively. The communication distance of the tag with the above sizes is, for example, about 3 to 10 m.
There is a tag intended to have a more compact size. For example, there is a tag in which the length of a dipole antenna is made shorter than λ/2 (for example, approximately 160 mm relative to a frequency of 953 MHz), and an inductance is formed to enable the dipole antenna to match with a chip. FIG. 22 depicts a diagram illustrating an exemplary configuration of such a tag 100. In the example depicted in FIG. 22, the length of dipole sections 103-1, 103-2 (X-axis direction in FIG. 22) is 73 mm, and the width (Z-axis direction in FIG. 21) is 7 mm. Here, the tag 100 depicted in FIG. 22 includes the dipole sections 103-1, 103-2, an inductance section 104 and a chip 105. Further, a tag antenna 109 includes the dipole sections 103-1, 103-2 and the inductance section 104, for example.
The chip 105 in the RFID may be represented equivalently as a parallel circuit composed of a capacitive component Cc=1.0 pF and a resistive component Rc=1750Ω. Meanwhile, the dipole sections 103-1, 103-2 may be represented equivalently as a parallel circuit composed of a resistor Ra and an inductance La. FIG. 23 depicts a diagram illustrating an example of the equivalent circuit of the tag 100 depicted in FIG. 22. The equivalent circuits of the chip 105 and the dipole sections 103-1, 103-2 are connected in parallel, and the inductance La of the dipole sections 103-1, 103-2 is determined, for example, in a manner to be resonant with the capacitor Cc of the chip 105 (for example, f0=2π/√(LaCc)). By this, the chip 105 matches with the dipole sections 103-1, 103-2 for example, at a desired frequency f0 (for example, 953 MHz or the like), so that the reception power of the dipole sections 103-1, 103-2 may sufficiently be supplied to the chip 105 side.
As such, the tag 100 is designed in consideration of a matching condition etc., and the tag 100 in the RFID system is used by being affixed to a some dielectric (dielectric constant ∈r, thickness t (Y-axis direction in FIG. 22)). Therefore, at the design of the tag 100, the sizes of the tag antenna 109 etc. are designed in consideration of the dielectric constant ∈r and the thickness t of an affix object.
The tag 100 depicted in FIG. 22 represents an example when the tag 100 is affixed to an affix object 101 (for example, plastics such as a polycarbonate and an ABS resin (acrylonitrile-butadiene-styrene copolymer synthetic resin)) having dielectric constant ∈r=3 and thickness (Y-axis direction in FIG. 22) t=10 mm.
FIG. 24 is a drawing obtained by executing an electromagnetic field simulation on the tag 100 and plotting on an admittance chart each calculation result when a frequency (for example, a radio signal frequency transmitted and received by the tag 100) f is varied from f=700 MHz to 1200 MHz. The simulation is based on a case such that an affix object 101, of which dielectric constant ∈r is ∈r=3 and thickness t is t=10 mm, is affixed to the tag 100. The chip 105 may be represented equivalently as a parallel circuit of which resistance Rcp is Rcp=1750Ω and a capacitance Ccp is Ccp=1.0 pF, and on the admittance chart, the chip 105 is plotted at a point represented by an open circle. A point obtained by reversing the ± of an imaginary component relative to the position (open circle) of the chip 105 becomes an optimal point (filled circle). At the above optimal point, the imaginary component of the chip 105 and the imaginary component of the dipole sections 103-1, 103-2 have the same magnitude, to cancel each other, and thus, the dipole sections 103-1, 103-2 may resonate with the chip 105.
The bold broken line depicted in FIG. 24 represents a locus (“dipole shorter than λ/2”) of the tag 100 to which the affix object 101 is affixed when there is no inductance section 103. In this case, an imaginary part becomes 0 when the radiation resistance Rap of the dipole sections 103-1, 103-2 is Rap=72Ω.
The thin broken line depicted in FIG. 24 represents a locus (“minute dipole with inductance”) when an inductance section 103 is connected to the tag 100 to which the affix object 101 is affixed. The obtained locus (thin broken line) of the tag 100 results after counterclockwise rotation of the locus (bold broken line) having no inductance section 104 as a whole. In the tag 100 to which the inductance section 103 is connected, a position having an operation frequency f=953 MHz is represented with a triangle in FIG. 24, which is overlapped with the optimal point. Therefore, it is possible to consider that an optimal size of the tag 100 relative to the affix object 101, having dielectric constant ∈r=3 and thickness t=10 mm, is as represented in FIG. 22, for example.
However, the tag 100 is not always affixed to the affix object 101 having the identical dielectric constant ∈r and thickness t. The tag 100 may be affixed to an affix object having a different dielectric constant ∈r and a different thickness t from those of the affix object 101.
FIG. 25 depicts graphs representing examples of a frequency characteristic with regard to a communication distance when the thickness t of the affix object is changed without change of dielectric constant ∈r (∈r=3). In the graphs, the dotted line represents a case when the thickness t is t=10 mm, the solid line represents a case when the thickness t is t=20 mm, and the bold line represents a case when the thickness t is t=2 mm, respectively. Such graphs are obtained by the execution of the electromagnetic field simulation. The communication distance becomes maximal at a desired frequency f0 (for example, f0=953 MHz) when the thickness t of the affix object is t=10 mm (the affix object in this case is the affix object 101).
As depicted in FIG. 25, when the thickness t of the affix object is thinned from 10 mm to 2 mm, a frequency producing the maximum communication distance is shifted from the desired frequency f0 to the high frequency side. On the other hand, when the affix object thickness t is thickened from 10 mm to 20 mm, a frequency producing the maximum communication distance is shifted from the desired frequency f0 to the low frequency side.
A reason that the frequency producing the maximum communication distance is shifted from the desired frequency f0 to the high frequency side when the affix object thickness t is thinned is, for example, as follows: If the affix object thickness t is thinned from 10 mm to 2 mm, as the thickness is smaller, so a region having the dielectric constant ∈r=3 becomes smaller (or a region having the dielectric constant of air ∈r=1 becomes larger), and thereby an effective dielectric constant ∈e becomes smaller. With regard to the relationship of the effective dielectric constant ∈e to a wavelength λ, for example, the following equation holds.λ=λ0/√(∈e) (where ∈0 is the length of one wavelength in a free space(for example, air))  (1)
Therefore, if the effective dielectric constant ∈e becomes smaller, the wavelength λ0 of a radio signal propagating through the air is shrunk and shortened. With regard to the relationship of the wavelength λ to the frequency f, for example, a relational expression ofc=fλ (where c is light velocity)  (2)holds, and therefore, if the wavelength λ0 of a radio signal propagating through the air is shortened, the frequency f is increased. In other words, if the thickness t of the affix object is thinned from 10 mm to 2 mm, a frequency producing the maximum communication distance is shifted from the desired frequency f0 to the high frequency side.
On the other hand, if the affix object thickness t is thickened from 10 mm to 20 mm, because of a reason contrary to the reason in the case of thinned thickness t from 10 mm to 20 mm, a frequency producing a maximum communication distance is shifted to the lower frequency side than the desired frequency f0.
Here, a similar result is obtained when the dielectric constant ∈r is changed without change of the thickness t of the affix object. Namely, if the dielectric constant ∈r is decreased from “3” to “2” without change of the affix object thickness t, the effective dielectric constant ∈e is decreased, and then, from the relationship of expression (1) and expression (2), a frequency producing the maximum communication distance is shifted from the desired frequency f0 to the high frequency side. On the other hand, if the dielectric constant ∈r is increased from “3” to “4” or the like, without change of the affix object thickness, oppositely, a frequency producing the maximum communication distance is shifted to the lower frequency side than the desired frequency f0.
As such, when the dielectric constant ∈r and the thickness t of the affix object 101 are changed, the frequency producing the maximum communication distance is shifted either to the high frequency side or the low frequency side, and the communication distance at the desired frequency f0 is reduced as compared to that before the change. If the communication distance becomes shorter than the maximum, undesirably the capability of information readout such as ID from a tag becomes smaller.
In consideration of the above-mentioned problem, there is a technique related to the RFID as described below, for example. Namely, with the provision of an auxiliary antenna capable of contacting to or capacitive coupling with a metal portion of the tag antenna in a radio tag vessel, so as to virtually change the length of the tag antenna, the frequency characteristic of a tag antenna may be tuned.
Further, in a non-contact data carrier including an IC chip and a closed-loop antenna, there is a technique for adjusting the dispersion of a resonance frequency by overlaying a conductor such as a metal plate with the closed-loop antenna.
Additionally, in domestic Japan, the Ministry of Internal Affairs and Communications is determined that, by the year 2015, the transition of a frequency for use for the RFID system is to be made from the current frequency band of 952 MHz to 954 MHz to a frequency band of 915 MHz to 927 MHz.